Police repeatedly fired tear gas in clashes with protesters fighting for democracy as parts of Hong Kong descended into chaos Sunday with tens of thousands on the streets to demand Beijing grant the city full universal suffrage.

The rare scenes -- in which crowds faced down riot police in the international financial hub -- forced protest leaders to warn supporters to "retreat and save their lives" if rubber bullets were fired.

Protesters screamed "Shame!" at officers, many in gas masks and riot gear, as they tried to shield themselves from the clouds of tear gas which was last used in Hong Kong in 2005.

It marked a dramatic escalation of protests in the city, which rarely sees such violence, after a tense week of largely contained student-led demonstrations exploded into mass angry street protests.

Protesters have defiantly stuck to their demands for full universal suffrage after Beijing last month said it would allow elections for the city's next leader in 2017 but will vet the candidates -- a decision branded a "fake democracy".

An AFP reporter at the scene early Monday morning estimated ten thousand protesters were dug in for another night as the unrest spilled over into other areas beyond the main site for the first time -- with thousands launching a sit-in across the harbour.

Protest leaders on Sunday called on demonstrators to pull back if police used rubber bullets, with rifles slung over the shoulders of many officers, or if they felt their lives were threatened.

"This is a matter of life or death. If their lives are threatened they should retreat and save their lives," said professor Chan Kin-man, a co-founder of the Occupy Central group which threw its weight behind the protest on Sunday.

As acrid plumes of smoke wafted across the city, demonstrators expressed outrage at the city's police but remained defiant. 

"We are unarmed, just standing here, there was no warning for the gas," Harry Hung told AFP after a volley was fired.

"This is unbelievable. This is a peaceful protest and the police are the ones using violence," added demonstrator Jade Wong. "The level of police violence here is just like mainland China, it was never like this before."

The tear gas did little to stem the tide of demonstrators occupying more than 800 metres of a vital multi-lane highway usually filled with vehicles.

Fresh protests also sprung up far from the main demonstration, with around 3,000 people blocking a major road across the bay in Mongkok, bringing traffic to a standstill and opening up another significant flashpoint.

A dozen police looked on helplessly as the demonstrators took over the bustling Nathan Road in Kowloon, chanting slogans and wrapping their eyes in clingfilm to protect themselves in the event of pepper spray being deployed, an AFP reporter said.

"We need to strike for freedom and for our democracy. We've come to Mongkok because it's very dangerous now in Central and Admiralty," 20-year-old student Calvin Chan told AFP, referring to the main protest districts.

Also affected was the busy shopping district of Causeway Bay, east of central Hong Kong, where some 1,000 people facing a line of riot police.

Officers have so far made 78 arrests for offences ranging from forcible entry into government premises, unlawful assembly, disorderly conduct in public place and assaulting public officers.

Just hours before people were due to start work, the police issued a statement urging protesters to "stay calm, stop charging police cordon lines and occupying the main roads, so that the roads can be re-opened to emergency and public vehicles".

Twenty-six people were being treated for injuries, the hospital authority said.

China, which stations a People's Liberation Army (PLA) garrison in Hong Kong, said it was confident the city's administration could handle the protest.

Beijing "firmly opposes all illegal activities that could undermine rule of law and jeopardise 'social tranquility' and it offers its strong backing" to the Hong Kong government, said a spokesman for China's Hong Kong and Macau affairs office, Xinhua news agency reported.

In a statement the Hong Kong government said it had "no intention to seek help from the PLA".

Students have boycotted classes in the past week, which saw the central government complex stormed, with pro-democracy group Occupy Central on Sunday bringing forward a mass civil disobedience campaign that had been due to start on October 1.

Hong Kong's leader, Chief Executive Leung Chun-ying, told a press conference his administration was "resolute in opposing the unlawful occupation actions by Occupy Central".

As the demonstrations continued into Monday -- the start of the working week, authorities were forced to shut schools in key areas.

The Hong Kong Education Bureau announced that schools in the Wan Chai as well as Central and Western districts would be closed.

Former colonial power Britain handed Hong Kong back to China in 1997 under a "one country, two systems" deal that guarantees liberties not seen on the mainland, including freedom of speech and the right to protest. (AFP)